Imagers reproduce an image by converting photons to signals that are representative of a scene being imaged. In an imager, photosensors capture the photons and provide respective signals that represent a captured image. Each photosensor may include a photodiode, which is initially reset to a reference voltage, and which, after reset, integrates a charge over a specified time period proportional to the intensity of light on the photosensor. However, a relatively high light intensity may cause the photosensor to saturate, such that the accumulated charge is equal to the maximum amount of charge which can be held by the photosensor. Saturation therefore results in inaccurate reproduction of the image. Saturation can be mitigated by decreasing the integration time at the expense of image loss in darker areas of an image.
Attempts have been made to provide readout techniques that compensate for photosensor saturation while trying to preserve detail in darker areas of an image. However, conventional techniques often compromise other performance characteristics, such as frame rate, power consumption, etc., or may require more components, resulting in greater power consumption, and/or a consumption of greater die area, resulting in higher cost.
Moreover, conventional techniques may accurately represent the bright areas of an image while sacrificing contrast in the dark areas, or accurately represent the dark areas of the image while clipping the bright areas. However, conventional techniques often are not capable of accurately representing the bright and dark areas at the same time.
Thus, a method and apparatus are needed that enable an image sensor to obtain a relatively high dynamic range, as compared to a conventional image sensor, without compromising other performance characteristics, such as slower frame rate, greater power consumption, greater number of components, and/or higher cost, etc.
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